1. Field of the Invention
This invention relates to field sensors in general and to semiconductive magnetic sensors in the general field effect transistor type in particular.
2. Prior Art
A great amount of effort has been expended in developing field effect transistor magnetic sensor devices. The general operation of the prior art devices has consisted of utilizing what was previously understood to be Lorentz deflection of the carriers flowing between the source and one or more drains in a field effect transistor. This was used to create an imbalance in the drain currents thereof to give a differential signal output at the drains. See, for example, IBM Technical Disclosure Bulletin, Volume 13, No. 12, May 1971, page 3633, or, for example, the U.K. Pat. No. 1,243,178 in which an FET magnetic sensor having two or three drains and a source at opposite ends, respectively, of a field effect transistor channel is shown. As will be understood by those of skill in the art, suitable gate and insulating oxide layers are also formed in such devices to control the flow of current in the channel. In the aforementioned U.K. patent, by application of suitable voltage to the gate an inversion layer acting as a conductive channel is established. The effective conducting channel thus formed, extends between the source and two or three drains. The source is connected to ground and the drains are connected to a supply voltage through resistive loads which may be of equal value. As is well known in the art, when the device is operated with appropriate voltages on the source, on the drain and on the gate, a current will flow between the source and drains. It has been thought that this current stream could be deflected by Lorentz force produced by a magnetic field intersecting the path of the charge carriers. This is the well-known state of the art and, as stated in this U.K. patent, it is believed that the Lorentz deflection force will cause charge to "build up" on one side of the channel and to be depleted on the other side until the electric field produced by the displacement of charge applies a force on the charge carriers which is equal and opposite to that due to the magnetic field.
This classical teaching of a "charge buildup" occurring in the channel to oppose deflection of the charge carriers has resulted in the use of relatively wide channels in the field effect transistor structures of this type. This was to provide room to deflect the charge carriers without interaction or "charge buildup" occurring to a significant degree. Of course, a wider channel will carry proportionately greater currents than narrower channels would have. In the differential mode, only a small number of the current carriers (those near the center of the channel) are actually deflected in their path to strike one drain instead of another. The majority of carriers in the wide channel device will, therefore, add to the drain current at the output drains in a manner which is not affected by Lorentz deflection. This in turn means that noise voltages produced in the load resistors will be greater than would otherwise be the case with a narrow channel device. Furthermore, since only a relatively small proportion of the carriers in the channel are capable of being deflected to produce a signal, the signal levels produced relative to the noise levels produced will be relatively smaller than might be desired. Also, because of the greater relative width of a channel of this type, the resolution of the device, i.e., the narrowest band of magnetic flux which can be utilized to deflect the carriers that produce an output signal will be reduced.
The use of wide channels, i.e., those in which the channel width is greater than the width of the smallest drain or source of spacing of elements, indicates that the width of the depletion zones along the channel boundaries was either ignored or unrecognized in the prior art. In either event, the prior art depletion zones in such wide channel devices constitute an insignificant portion of the prior art channel width. In contrast, in the present invention, the widths of the depletion zones within the channel constitute a significant portion of the total channel width because the channel widths are so small that the depletion zones form a relatively greater portion of the total width.
Related prior art of a slightly different sort uses bipolar (P-N) or junction devices in which the source and one or more drains are of dissimilar conductivity material embedded in a substrate of semiconductive material. See, for example, U.S. Pat. Nos. 3,714,559 and 3,829,883 or 3,167,663 and 3,731,123. As noted in these patents, a major problem associated with magnetic field sensors of the FET type is the difficulty of obtaining high sensitivity with sufficiently large output signals under acceptable signal-to-noise ratios in sufficient bandwidth conditions. In the first mentioned U.S. Pat. Nos. 3,714,559 and 3,829,883, a multi drain FET magnetic field detector is shown which is operated in a mode in which the gate is biased to less than the transistor threshold and the first drain is biased to produce avalanche breakdown (a notorious noise source in FET's) of the junction with the substrate while the second and third drains are biased to a voltage below that required for avalanche breakdowns of their junctions. In this mode of operation the field effect transistor channel has not effectively been turned on to conduction. Therefore a field effect transistor structure is not essential for operation of this device since it does not operate as a field effect transistor in this mode and no "channel" exists. An alternative embodiment is shown in these patents in which at least one of the drains is of the same conductivity type as the substrate but more highly doped. This device is operated to produce a current between the source diffusion and the junction so that the device apparently operates more as a diode. These devices claim to have high sensitivity, relatively high signal output and good signal-to-noise ratios but are, as stated herein, not operated as field effect transistors. It is believed that they operate to accelerate holes diverted by a magnetic field and that these minority carriers in a deflection mode have improved sensitivity. The difficulties of avalanche breakdown control and generating sufficient hole-electron pairs to enable a hole current flow from the source to the drain may prove undesirable in certain applications. It is more desirable to use the majority carrier current, if possible, in field effect transistor structures operated as transistors.
The aforementioned U.S. Pat. Nos. 3,167,663 and 3,731,123 show other magnetic field detecting devices of the P-N junction type (bipolar). These devices operate essentially as diodes in which current flow can be changed or directed by the application of external magnetic fields. These devices are thought to be operated in the region of high current flow in which injected carriers are deflected by Lorentz deflection to result in lateral displacement of the carrier stream and a differential output signal between two or more P-N junction drains. These devices have intrinsically higher currents and, while they may be sensitive to magnetic fields, they are also more subject to noise current phenomena.
A similar type of device to the previously discussed patents is shown in U.S. Pat. No. 3,593,045 in which a beam of injected electrons created at a P-N junction in a semiconductor device is supposed to be deflected to one or more targets by electric or magnetic fields. This device is not operated as a field effect transistor, however, and, unfortunately, requires provision of relatively high biasing and driving voltages, typically on the order of 200 volts as described in the patent, which make it unsuited for application to field effect integrated circuits.
Another area in which the prior art has provided some development and investigation is in the field effect Hall devices such as that illustrated in U.S. Pat. No. 3,448,353, for example. These devices, of which U.S. Pat. No. 3,448,353 is typical, do not directly utilize the deflection of carriers in a field effect device but utilize the offset voltage produced by Lorentz deflection of equipotential lines of the carriers transverse to the input and output connections. The Hall devices are usually characterized as well as relatively low length-to-width ratios (under approximately three to one) and are best operated with equal width and length as is now well known in the art. The output signal in Hall devices, as is also well known, is proportional to carrier velocity and not to the number of carriers. Since these are not field effect transistor structures operated in the mode in which a beam of carriers is deflected and a differential output signal at the drains obtained, only a voltage output from the Hall output probes on either side of the channel can be used. It is preferable to have an actual signal current obtained at one or more of the drains as is the case with the aforementioned differential deflection field effect structure such as described in the aforenoted U.K. patent.
Still other related prior art is in the field of charge coupled devices which control the state of current flow in field effect transistor channels of relatively greater width such as that shown in U.S. Pat. No. 3,714,523 or in the IBM Technical Disclosure Bulletin, Volume 14, No. 11, April 1972, page 3420. The patented device provides high sensitivity for magnetic field detection. A differential control of the gate electrodes coupled back from the drains may be utilized to provide positive feedback to create extremely sensitive devices as described in the patent. Charge coupled devices of this type also provide an amplification factor and may be utilized to provide higher output signals. However, due to the relatively greater width of such devices and greater current flows, higher noise output voltages may be expected. Such noise is amplified by the feedback mechanism used, as well. Of course, the great width of these devices also indicates that the depletion zones on either side of the channel are an insignificant portion of the total channel width.